As one of the prominent earlier efforts to understand quantum-entanglement properties of the H atom, Tommassini et al. [1]} have analyzed the electron–proton coordinate entanglement, understood as correlations in the electron–proton motion in the H atom. While the behavior of this electron–proton position entanglement as a function of temperature is yet to be understood, here, we focus on another type of quantum entanglement built into the H atom – entanglement related to the nuclear and electron spin degrees of freedom. An illuminating analysis of entanglement of this type has been earlier presented by Zhu et al. [2]}, who developed a helpful formalism for the description of electron–nuclear spin entanglement in the H atom and examined important properties of his entanglement. Entanglement involving nuclear degrees of freedom is in no way unique to the H atom. As one prominent example, nuclear spins in nitrogen–vacancy centers in diamond have been shown to offer a powerful resource for quantum information, attracting much interest in the context of rapidly growing quantum technologies [3]}, [4]}, [5]}, [6]}.
